1. Field of the Invention
This invention relates generally to drum type continuous flow aggregate processing apparatus and methods. The invention is advantageously applicable to continuous flow soil remediation apparatus and methods, and is also advantageously applicable to such apparatus and methods using dual temperature zones.
2. Discussion of Related Art
Continuous processing of aggregate materials in drum driers and mixers has long been used in the production of road surfacing materials, such as hot asphalt aggregate mixes. Using similar drums for continuous processing of chemically contaminated soil to restore the soil to an environmentally acceptable condition has recently come into use.
Plant installations for remediating soil in continuous flow processes provide the means for cleaning up soil contaminations resulting from leaking underground storage tanks of gas stations, for example. Recent advances in the soil remediation technology have shown that flow-through or continuous process apparatus for dry heat soil decontamination and remediation operations is a feasible alternative to known batch type operations and particularly to wet processing batch type cleanup operations. In the course of being decontaminated or remediated, the soil materials to be cleaned of hydrocarbon type chemicals are heated to the volatization temperature of the contaminating hydrocarbons. One of the energy concerns in such a process is that hydrocarbon contaminants become vaporized over a rather broad range of temperatures. Therefore, to rid the soil of all hydrocarbons necessitates that it be heated to the highest temperature at which all hydrocarbons have become vaporized.
Mixes of hydrocarbons of differing chain-lengths cause the temperature requirements to vary. Vaporization temperatures vary depending on whether spills of heating oils or gasoline spills are to be removed from the soil. A soil remediation plant may need to accommodate hydrocarbon contaminated soils which become decontaminated only when heated to at least one thousand degrees Fahrenheit. Other contaminated soils may readily be cleaned of contaminating chemicals at temperatures no higher than five hundred degrees Fahrenheit, for example. Though it is possible to subject all soil with hydrocarbon contamination consistently to an uppermost required temperature at which even the most resisting contaminants become volatilized, the energy required to heat the bulk of the materials consistently to temperatures higher than necessary is wasteful and may itself contribute to unnecessary air pollution. It is furthermore undesirable to have a remediated soil product which is discharged at temperatures much higher than necessary to remove contaminants, and which then requires substantial time to cool before it is handled in further operations.
Energy requirements for effecting soil remediation also are found to vary with the coarseness of the material that is being decontaminated. Stone, or coarse aggregate, which is considered to be a high-capacity heat storage material and which is also a relatively poor heat conductor, requires a substantial energy input to become heated to a temperature at which hydrocarbon retained in its crevices will vaporize out. Energy requirements are further complicated because in many instances "fill dirt" containing substantial amounts of coarse materials may have been used to install some underground storage tanks. When the soil is returned to excavation sites after remediation, it is desirable to selectively replace the remediated soil to provide surface soil with a minimum of or with no coarse aggregate content while subsoil may contain a much greater or a substantial amount of such coarse aggregate.
Another problem or consideration concerning flow-through decontamination apparatus or processes for soils pertains to the cleanup of hot gases which perform the soil remediation process and which are subsequently released to the atmosphere. Vaporized hydrocarbons tend to cause blue smoke if simply vented to the atmosphere without further processing. According to currently known practices, vaporized hydrocarbons are routed toward the flame and are burned therein to reduce air pollution to acceptable standards, and to contribute at the same time toward the energy required for generating the hot gases which heat the soil material.
Because of the various factors that can affect the success of a commercial soil remediation operation, further improvements in processes and apparatus are desirable. Improvements are needed which address the above discussed problems and, at the same time, take into consideration the need to maintain compactness and cost efficiency in a soil remediation apparatus. These improvements would desirably further optimize energy requirements of such an apparatus for remediating soil of various aggregate sizes and conditions.